Just stating the obvious ...
Netflix has recently restructured its services so there's now 4K subscription tiers, if you aren't paying extra for 4K then they aren't allocating more bandwidth to deliver 4K. If you just signed up for your UltraHD then it might take a little while for it to kick in.
And the not-so-obvious ...
Many ISPs implement some type of "Wireless Experience Video Optimization" which will conserve bandwidth by compressing streaming video content into smaller resolutions and lower qualities when they detect (or assume) your hardware doesn't support greater capabilities. Mostly for mobile device users. But it can interfere with more robust platforms. Details vary from ISP to ISP and from region to region so you gotta google around to determine if it's an issue (and if there's a fix) for you.
Test if any other wired and wireless devices on your network can stream 4K, if the problem only affects one machine then you know it's not your gateway/router or anything else upstream on the network. If you can "borrow" somebody else's known-to-work 4K Netflix login for a few quick tests then you'll know whether or not it's your service account. You can test by streaming some 4K vids off YouTube to determine whether your machine has some hardware/driver issue preventing 4K playback. Test a different DP cable if nothing else works.
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